1. Technical Field
The present invention is directed to a power system and more particularly to a method and apparatus for providing post-regulation for a switch mode power supply with multiple outputs.
2. Description of Related Art
Integrated circuits continue their trend toward higher transistor densities and smaller feature sizes. As the technology for the various devices drifts toward different power supply requirements, various low voltage standards have been established. While early xe2x80x9clogic circuitsxe2x80x9d used 5V, today""s devices require 5V, 3.3V, 2.8V, 2.5V, 2.0V, 1.8V, 1.5V, 1.2V, 0.9V and others. Consequently, mixed low voltage systems have become commonplace. The packaging density and thermal demands have likewise continued to grow with each new generation of product. As a result, there is a need for power converters with high density, high efficiency, and multiple outputs with independent regulation, to energize these systems.
Another demand for a multiple output power converter is flexibility. Because few applications need exactly the same combination of output voltages and output currents, a successful catalog product should address a broad set of applications. This versatility can be achieved through adjustable output voltages and flexible output loading.
During the development of power supplies with the needed flexibility, several approaches have been suggested in the prior art. Some approaches have used linear regulation, but such a technique results in low efficiency especially if wide input voltage variations are present and is limited to power converters in which the load current of the auxiliary output is relatively low. A magnetic amplifier output regulator has also been used as a means for regulating more than one output of a switching supply. However, the magnetic amplifiers tend to be bulky, expensive, lossy (especially where the switching frequency is high), and have a limited control range in that minimum delay times reduce the maximum achievable output voltage for the auxiliary outputs. Cross regulation is another approach that has been suggested. This approach uses the winding ratios in the transformers to set the ratio between the main output and the auxiliary output. The drawbacks of cross regulation techniques include poor regulation, poor resolution in ratio selection, and no independent output adjustment. Yet another approach has been to connect one or more buck converters to the output of the main converter to deliver independently regulated outputs. However, this approach results in noise reflected back to the main output. This noise problem can be avoided if the auxiliary outputs are derived from the pulsating voltage at the secondary side of the transformer rather than from the output of the main converter. This most recent approach is commonly referred to as switching post regulation.
Phase modulation techniques have also been applied to switching post regulators to regulate the auxiliary outputs. Both leading edge and trailing edge modulation techniques have been suggested in the prior art. While trailing edge modulation (leading edge synchronization) is a viable option, it complicates the use of primary-peak-current-mode control of the main output. During trailing edge modulation, the termination of the post-regulator pulse results in a current signal in the primary switch with a peak value that does not necessarily occur at the end of the duty cycle pulse. This can result in current control instabilities. Similarly, primary-side peak current limit can be complicated with trailing edge modulation, using either peak current control or voltage mode control because there are two current peaks in the inductor current making it hard to detect the proper peak to use for current limit. Leading edge modulation (trailing edge synchronization) simplifies primary-side peak current limit by assuring that the current level at the end of the power delivery cycle is at its peak value.
A typical example of a leading edge modulation approach is a secondary side post regulator controller for DC to DC multiple output converters manufactured by Cherry Semiconductor Corporation (now a part of ON semiconductor) and identified under the product No. CS5101. A description of this product is found in Cherry Semiconductor Corporation""s xe2x80x9cSecondary Side Post Regulator for AC/DC and DC/DC Multiple Output Convertersxe2x80x9d dated March 1997 and incorporated herein by reference as if fully set forth at length. The post regulator control circuit ensures that the trailing edges of the main and auxiliary outputs are synchronized. A ramp is generated and triggered at the start of the main power delivery cycle and the turn-on of the synchronous switch. Depending on the error of the output voltage of the auxiliary output, a delay between the start of the main power cycle and the turn-on of the synchronous switch is generated. Leading-edge modulation of the auxiliary outputs is achieved. While good efficiency figures, good regulation, and low output noise can be achieved with this scheme, there is still a control circuit propagation delay between the detection of the start of the power cycle and the turn-on of the controlled forward rectifier. This limits the range of the auxiliary outputs because of the duty cycle loss of the auxiliary outputs with respect to the main output. This problem becomes worse as the switching frequency is increased. Use of the noisy secondary winding waveform to synchronize and trigger the ramp can also cause undesirable jitter or instability.
Referring now to FIG. 1, a schematic diagram of a prior art leading edge modulation power converter is illustrated. This circuit is described in further detail in U.S. Pat. No. 6,222,747 to Rinne et al. entitled xe2x80x9cPost Regulation Control Circuit for a Switch Mode Power Supply with Multiple Outputs,xe2x80x9d issued Apr. 24, 2001 and incorporated herein by reference as if fully set forth at length. Rinne et al. teaches resetting the ramp generator by detecting the end of the power cycle. Rinne et al. also displays diode rectified main outputs in the preferred embodiments. While the Rinne approach offers advantages in increasing the regulation range of the post regulator, the post regulator is unable to maintain regulation during periods of light load on the diode-rectified main output because of the reduced duty cycle and discontinuous inductor current for the main output. Furthermore, though the use of diode rectifiers does not require drive timing for the main output, the efficiency of the converter is compromised when such diode rectifiers are used. For the diode-rectified converter disclosed by Rinne et al, discontinuous inductor current cannot be avoided at load currents near zero. Discontinuous mode will occur below the critical load current point because the diode rectifiers cannot allow negative current to flow. Thus, in discontinuous mode, the voltage transfer function becomes dependent on load. As the load on the main output decreases toward zero, the duty cycle must be reduced to maintain regulation of the main output. This reduction in duty cycle reduces the width of the power cycle available to the post regulators, eventually causing them to lose regulation. For a more detailed description of continuous vs. discontinuous mode, see the textbook entitled Modern DC-to-DC Switchmode Power Converter Circuits by Severens and Bloom, dated 1985 and incorporated herein by reference as if fully set forth at length.
Furthermore, Rinne et al. does not address the timing and drive of the main output rectifiers for the case where synchronous rectifiers are used in the main output. Rinne et al. also teaches sensing the end of the power cycle by sensing a noisy secondary winding voltage. As a result, this noise may undesirably couple into the post regulator ramp generator.
What is needed in the art is a post regulator architecture for a power converter that offers full regulation range even during periods of light load on the main output. The architecture should provide means for synchronizing and providing a precise drive for all of the secondary rectifier switches in both outputs. The architecture should eliminate the noise coupling problems found in the prior art while providing an efficient and inexpensive regulation of the main and auxiliary outputs.
The present invention is an architecture for a post regulator control circuit that utilizes an advance trigger signal to trigger the post regulator ramp. This advance trigger signal anticipates the beginning and/or end of a power cycle, and can be used to drive all of the secondary rectifier switches with optimal timing to minimize both cross conduction and body diode conduction. The architecture can be used to cascade an arbitrary number of post regulators. The present invention provides to the auxiliary outputs the full range of regulation available to the main output even in light load conditions. Rather than sensing the beginning or end of the power cycle, the present invention anticipates the beginning and/or end of the power cycle using the pulse train generated by the feedback loop for the main output. This allows the circuit to prepare the switches for the beginning of the power cycle and avoids problems encountered with inherent propagation delays in the circuit. Using the advance trigger signal, all of the switches may be driven with precise timing. Because the advance trigger is not subject to the high currents or leakage inductance ringing associated with the power train operation, it provides a signal with much lower noise and more predictable timing.